Auraria Membership

LONG-TERM PERFORMANCE OF GEOSYNTHETIC REINFORCED SOIL
RETAINING WALLS
by
Phillip E. Crouse
B.S., University of Wyoming, 1988
A thesis submitted to the
University of Colorado at Denver
in partial fulfillment
of the requirements for the degree of
Master of Science
Civil Engineering
1996

This thesis for the Master of Science
degree by
Phillip E. Crouse
has been approved
by
/.
/
Judith Stalnaker
Mojj 1 /
Date '

Crouse, Phillip E. (M.S., Civil Engineering)
Long-term Performance of Geosynthetic Reinforced Soil Retaining
Walls
Thesis directed by Professor Jonathan T.H. Wu
ABSTRACT
Since 1980, the use of geosynthetic reinforced soil (GRS) retaining walls
have become increasing popular due to their demonstrated distinct
advantages over conventional retaining walls. However, the lack of
quantitative data as led to the misunderstanding of the long-term
behavior of the soil/geosynthetic composite resulting in grossly
overconservative designs. In some cases, only 20 percent of the
strength of the reinforcement is allowed for designs due to concern
over potential creep in the geosynthetic element. The purpose of this
study is three fold. First, compile long-term quantitative data of full-
scale GRS retaining walls. Second, quantify the conservativeness of
the designs. Third, derive an approach for predicting creep based on
the behavior of the soil/geosynthetic composite. The quantitative
data compiled for the study suggest that reducing the strength of the
reinforcement based on the element creep tests results in grossly
overconservative designs. The approach developed for predicting
creep consists of using a simple soil/geosynthetic composite
laboratory creep test and analytical procedure to predict creep
based on project specific conditions.

This abstract accurately represents the content of the candidate's
thesis. I recommend its publication.

1. Introduction
Retaining walls have become an increasingly popular method for
retaining earth to accommodate worldwide development of transportation
and other structural systems. Conventional gravity and cantilever retaining
walls that externally resist lateral earth pressure can be costly and difficult to
build because of their large rigid mass. However, a new type of retaining
wall is available that derives its stability from within the backfill (i.e., is
internally stabilized) and is demonstrating distinct advantages over
conventional retaining walls.
In France, H. Vidal introduced modern applications of soil-reinforced
retaining walls in the 1960s (Vidal, 1966) using metal strips for reinforcement.
The idea of internally stabilizing soil is to strengthen the soil mass by the
inclusion of planar reinforcement whose function it is to restrain the
development of tensile strain in the direction of the reinforcement.
Reinforcement can be inextensible (e.g., metals) or extensible (e.g.,
geosynthetics). Since 1980, geosynthetics have been used for reinforcement
due to their flexibility and low cost. Soil reinforced with geosynthetics is
referred to as geosynthetic reinforced soil (GRS). Some of the advantages of
GRS retaining walls over conventional retaining structures include:
Their flexibility allows greater tolerance to foundation
settlement;
Construction of GRS walls is rapid and requires only
ordinary" construction equipment; and
GRS retaining walls are generally more economical than
conventional retaining walls.
The primary components in a GRS retaining wall include the
reinforcement, wall facing, reinforced soil backfill, retained soil, and

foundation soil. Figure 1.1 illustrates these components in a typical GRS
retaining wall.
Since the development of GRS technology, researchers have
identified three characteristics that are not well understood when reinforcing
soil with geosynthetic material. These include:
Lateral earth pressure distribution;
Failure surface; and
Creep.
This study focuses on creep in a GRS retaining wall. Lateral earth
pressure distribution and the failure surface have been addressed by several
other researchers and is ongoing.
2

HEIGHT
Figure 1.1
Components of a GRS Retaining Wall
3

1.1 Background
Since the mid-1980s researchers have attempted to characterize the
long-term behavior of GRS retaining walls. The overall research objective has
been to understand their long-term behavior to guide the development of
rational methods of analysis and design. Although this has been the overall
objective, researchers have approached the problem from three different
aspects:
Instrumenting full-scale GRS retaining walls;
Soil/geosynthetic composite laboratory creep tests; and
Element laboratory creep tests of geosynthetics.
Since the 1960s numerous full-scale GRS retaining walls have been
built and instrumented to quantify their performance. However, these walls
typically were monitored for relatively short periods of time due to financial
constraints and/or instrumentation damage. Since the late 1980s researchers
have built a few full-scale GRS retaining walls that have been monitored for
extended periods of time to quantify their long-term performance. The
results from these instrumented walls have been individually documented,
but have never been investigated in a unified manner.
In 1994 a soil/geosynthetic composite laboratory creep test was
developed by Wu (1994a) and Wu and Helwany (1996) to characterize the
complex behavior of the soil/geosynthetic composite. The test simulates the
composite by transferring stresses applied to the soil in a manner similar to
the typical load transfer mechanism in a GRS retaining wall. Ketcharf and
Wu (1996) continued the research by developing a simple test procedure to
assess the long-term behavior of GRS walls and tested various soils and
reinforcement materials under different conditions.
4

The current state of practice is to account for creep by performing a
creep test on the reinforcing element. Laboratory tests such as the
procedure contained in the American Society for Testing and Materials
(ASTM) D5262 Test Method entitled Tension Creep Testing of Geo textiles" is
used to determine a creep-limited strength of the reinforcement elements.
The test consists of applying a constant load for a minimum duration of
10,000 hours to an eight-inch-wide specimen. Because of the obvious time-
constraint of the test, estimated creep-limited strengths are typically used for
GRS retaining wall designs instead of performing the actual test. The creep-
limited strength is computed by applying a creep reduction coefficient
(CRC) or partial factor of safety to the geosynthetics' short-term strength.
Current American Association of State Highway and Transportation Officials
(AASHTO) design methods recommend reducing the short-term strength by
as much as 20 to 80 percent to account for creep.
The fundamental assumption in using results from geosynthetic creep
tests is that the soil/geosynthetic composite wall will behave the same as the
reinforcement element. However, results from full-scale and laboratory tests
have revealed that the geosynthetics perform significantly better when
confined in GRS walls than predicted by the element creep tests due to
stress redistribution in the soil/geosynthetic composite. Because of this
discrepancy, current design methods are overconservative and are inhibiting
the development of GRS technology.
1.2 Research Need
Since geosynthetics are creep-sensitive materials, designers are
concerned about providing adequate margins of safety to account for
creep in permanent GRS retaining wall applications. This, along with the lack
5

of quantitative long-term performance data has led to the misunderstanding
of the complex behavior of the soil/geosynthetic composite resulting in
overconservative designs. Therefore, the first research need is to compile
existing, quantitative, long-term performance data, from full-scale, well-
instrumented GRS retaining walls. The second research need is to develop a
rational method for estimating creep for the design life of the structure based
on the creep behavior of the soil/geosynthetic composite instead of the
geosynthetic element alone.
1.3 Research Objectives
The three main research objectives include:
1. Compile long-term performance data from field projects involving
well-instrumented GRS retaining walls;
2. Develope a means to quantify the conservativeness of the designs;
and
3. Develop a rational method to estimate creep based on laboratory
creep test of the soil/geosynthetic composite deformation
To meet the first objective, the following tasks were performed:
An extensive literature search was performed to determine what
projects could be used for the study of long-term performance;
A request for information was sent to experts in GRS technology;
Specific projects were selected for the study; and
Specific design and performance data from the selected projects
were compiled and summarized.
To meet the second objective, the following tasks were performed:
The actual or design creep reduction to the reinforcements' tensile
strength is compared to reductions recommended by AASHTO;
A conservatism index (Cl) was developed to quantify the
conservativeness of the design and;
6

A simple procedure was developed to predict creep using a simple
laboratory test and analytical equation can be used to predict
creep
To meet the third objective, the following tasks were performed:
The laboratory test procedure used to model the creep behavior of
the soil/geosynthetic composite was described;
The laboratory creep tests were validated using the performance of
the selected projects; and
A rational procedure was developed using the laboratory test and
analytical equation to estimate creep for the design life of a GRS
retaining wall.
1.4 Report Organization
Chapter 1 presents the introduction, background, research needs and
research objectives. Chapter 2 describes the projects selected from the
literature survey. Chapter 3 describes the design and long-term
performance of the selected projects. Chapter 4 describes the method to
estimate creep using the laboratory soil/geosynthetic model creep tests.
Chapter 5 describes the conclusions and recommended future research.
Appendix A contains the selected project descriptions. Appendix B contains
the conservatism index computation and Appendix C contains the graphs
used to compute the creep modulus.
7

2. Literature Review and Survey of Creep Performance in GRS Retaining Walls
Since the 1960s researchers have built and instrumented numerous
full-scale soil-reinforced retaining walls to quantify their performance.
However, due to financial constrainfs and/or instrumentation damage,
researchers could monitor the wall performance for only relatively short
periods of time. In the 1980s, transportation officials began using GRS
retaining walls for highway and railway renovation projects and sponsoring
research in GRS technology. With support from the transportation resources,
researchers installed instruments in some of these walls to monitor their long-
term performance under actual service and field conditions.
In this study, an extensive literature review and survey was conducted
to collect information on the projects that used GRS retaining walls that had
been monitored for extended periods of time (i.e., greater than six months).
A survey was developed and sent to 10 internationally renowned experts to
obtain information on GRS projects under their direction. From the literature
review and survey, seven GRS retaining wall projects were selected. These
projects typically had well-documented, long-term reinforcement strain
data, wall deformation data, and design data. The projects selected are
listed in Table 2.1. The locations of fhe projects are illustrated on Figure 2.1.
8

The walls built for each project represent a variety of GRS retaining
walls. The walls range from 15 feet to over 40 feet in height and typically
include surcharge loads comprised of earth fills or highway loads.
Reinforcement materials consist of polypropylene or polyester geogrids and
geotextiles ranging in short-term strength from 400 to over 12,000 lb per foot
width. The facing used on the walls consists of concrete modular blocks and
panels or exposed surfaces. Some of the walls are constructed on poor
foundations while others are constructed on competent foundation
materials. The environmental conditions vary from freezing temperatures in
Ontario, Canada, to temperatures up to 1110 Fahrenheit for walls built in the
state of Arizona, USA.
Although the selected projects consist of a variety of GRS retaining
wall types, all the walls performed exceptionally well. The maximum strains
measured in the reinforcement in all cases were less than five percent. In
some cases, the designs predicted strains of 40 to 60 percent. In other cases,
the walls were designed to fail, yet failure could not be achieved. The
following sections provide a brief description of the selected projects and
design approach. Chapter 3 provides the performance evaluation.
2.1 Project Descriptions
The following sections provide a brief overview of the projects
selected from the literature review and survey. The GRS retaining walls built
for each project are illustrated on Figure 2.2. Selected project information is
provided on project description sheets in Appendix A. The project
description sheets include information such as the wall components (i.e.,
confining soil, facing, and reinforcement type), reinforcement strength,
surcharge, and schedule showing dates of milestone events such as the
11

beginning of construction, surcharge loading, and monitoring period. A
schematic of the retaining wall(s) and project photographs are also included
on the project description sheets.
2.1.1 Interstate Highway 70 through Glenwood Canyon Project
In April of 1982, the Colorado Department of Highways designed and
constructed a series of internally reinforced walls for the Interstate Highway
70 project through Glenwood Canyon. The Glenwood Canyon follows the
Colorado River through the scenic Rocky Mountains of Colorado, USA, near
the city of Glenwood Springs. The retaining walls were built over highly
compressible silts and clays at the base of the canyon. Because of
architectural and environmental constraints, transportation officials tested a
series of internally reinforced retaining walls including a reinforced earth wall,
retained earth wall (VSL), a wire-mesh reinforced wall, and a geotextile-
reinforced wall. The geotextile reinforced wall was one of the first full-scale
GRS walls constructed in the USA.
The performance of the GRS retaining wall was observed for several
years; however, quantitative performance data was documented for only
the first seven months of service. The wall was designed to determine the
lower stability limits of a GRS retaining wall, therefore geotextiles having
relatively low tensile strengths (i.e., 400 to 900 Ib/ft) were used for the
reinforcement. In June, 1983, a 15 foot high surcharge was applied to the
top of the wall in an attempt to collapse the wall. However, failure never
occurred.
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In 1983 and 1993, samples of the reinforcement were exhumed to
determine the survivability and durability of the reinforcement (Bell and
Barrett, 1994). The strength of the exhumed reinforcement was compared
with that of archive samples. The results of the test are described in Chapter
3. Additional project information can be found in Federal Highway
Administration (FHWA) Report No. CHOH-DTP-R-86-16 entitled "Evaluation of
Fabric Reinforced Earth Wall" (Derakhsandeh and Barrett, 1986).
2.1.2 Tanque Verde Wrightstown Pantano Roads Project
In 1984 and 1985, 46 GRS retaining walls were constructed in the city
of Tucson as part of the Tanque Verde Grade Separation Project. In
September of 1985 two of the walls were instrumented (Wall Panels 26-30
and 26-32) to monitor their performance during and after construction.
Approximately seven years of performance data have been published for
the two instrumented walls (Collin, Bright, and Berg, 1994). The original
design and instrumentation information is contained in an Federal Highway
Administration (FHWA) report entitled Tensar Geogrid-Reinforced Soil Wall
(FHWA, 1989). Other papers have been written by Berg, Bonaparte, Anerson,
and Chouery (1986) and Fishman, Desai, and Sogge (1993) describing the
construction and performance of the walls.
The city of Tucson is located in the southern part of the state of
Arizona, USA, in the Sonora desert where summer temperatures can reach as
high as 111 Fahrenheit. Soil temperatures within the wall reached as high as
97 Fahrenheit. Elevated temperature environments for geosynthetics were a
potential design concern since the high temperatures may accelerate
mechanisms of degradation. Similar to the Colorado project, reinforcement
samples were exhumed after 11 years of service to examine the durability of
14

the reinforcement (Bright, Collins and Berg, 1994) which is described in
Chapter 3.
2.1.3 Norwegian Geotechnical Institute Project
In 1987, the Norwegian Geotechnical Institute (NGI) built a full-scale
GRS retaining test wall in Skedsmo, Norway. The purpose of the wall was to
establish characteristics of creep in the reinforcement. Skedsmo is located
near the city of Oslo, Norway, in northern Europe. The climate at Oslo is
moderate with temperatures ranging from 38 Fahrenheit in the winter to 64
Fahrenheit in the summer. Rainfall can be heavy at times with approximately
40 inches of rainfall annually.
The wall was instrumented in two sections, lJ' and N, each with a
different arrangement and spacing of the reinforcement. Approximately
four years of performance data have been published for the two
instrumented sections (Fannin and Herman, 1992). Following construction, the
wall was monitored for approximately four weeks under self-weight loading.
Thereafter, the top of the wall was cyclically loaded by using water tanks
that applied a maximum contact pressure of 6,000 lb/ft2. After
approximately two months of cyclic loading, the tanks were removed and a
permanent 10-foot-high surcharge was placed on top of the wall applying a
uniform and sustained pressure of 10,000 lb/ft2.
The original design and instrumentation information are contained in
the paper entitled Geosynthetic Strength Ultimate and Serviceability Limit
State Design" by Fannin and Hermann (1992). An additional paper
describing the project Fannin and Hermann, (1990) has also been published.
15

2.1.4 Japan Railway Test Embankment Project
Two test embankments were constructed at the Experiment Station of
Japan Railway Technical Research Institute near Tokyo, Japan. The test
embankments were part of a series of embankmenfs consfructed with sand
and Tokyo's sensitive clays in the 1980s to develop an internal reinforcing
sysfem thaf could withstand its heavy precipitation events (Tatsuoka,
Tateyama, Tamura, and Yamauchi). The first test embankment (JR Number
1) was backfilled with sand while the second embankment (JR Number 2)
was backfilled with clay. JR Number 1 was selected for this study.
JR Number 1 was constructed in 1988 to evaluate the stability of GRS
embankments with rigid facing. Instruments were installed during
construction and monitored for approximately two years until 1990, when it
was loaded to failure. The facing consisted of rigid cast-in-place concrete
panels installed in five wall segments. One wall segment consisted of
discrete panel squares for comparison with the rigid panels. The overall
project information can be found in a paper written by Tatsuoka, Murata,
and Tateyama (1992) entitled Permanent Geosynthetic-Reinforced Soil
Retaining Walls used for Railway Embankments in Japan".
2.1.5 Highbury Avenue Project
The Royal Military College of Canada has published several papers
documenting the long-term performance of a GRS retaining wall used in
reconstructing and widening Highbury Avenue in London, Ontario, Canada.
The wall was instrumented during construction in late 1989. Approximately 2
years of performance data have been published through August of 1991
(Bathurst, 1992). The research objective for the project was to collect
performance data from a well-instrumented in-service GRS retaining wall to
16

evaluate its long-term performance. Additional information can be found in
the paper by Bathurst (1992) entitled Case Study of a Monitored Propped
Panel Wall".
2.1.6 Federal Highway Administration Research Project
From 1984 to 1989, the FHWA sponsored several soil reinforcement
research projects at its stone quarry in Algonquin, Illinois, USA. One project
consisted of building a wall referred to as "Wall 9". The wall was built to
quantify the long-term behavior of continuous filament polyester geogrid
reinforcement and dry-stacked, soil filled facing units (Simac, Christopher
and Bonczkiewicz, 1990). The test wall was constructed with a very low
factor of safety to evaluate the applicability of existing design methods. The
internal stresses were monitored for three months, then an inclined surcharge
approximately seven feet high was place and monitored for approximately
1.3 years.
2.1.7 Seattle Preload Fill Project
In March of 1989, the Washington State Department of Transportation
designed and supervised the construction of a series of GRS retaining walls to
provide a preload fill in an area of limited right-of-way located in Seattle,
Washington, USA. The tallest wall (southeast wall) constructed for the project
had a height of 41.3 feet and supported 17.4 feet of surcharge fill. Since this
wall was significantly higher than any previously constructed wall,
instrumentation was installed to monitor its performance. The wall was
monitored for approximately one year after which it was demolished.
Specific design information can be found in the paper entitled Performance
of a 12.6 m High Geotextile Wall in Seattle, Washington" (Allen, Christopher,
and Holtz, 1992).
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2.2 Design Approach Evaluation
This section summarizes the approach used to design the GRS
retaining walls selected for the study previously described. The purpose for
evaluating the design approach is to illustrate how the current
methodologies address design considerations such as external and internal
stability, creep, construction damage, and biological degregration of the
reinforcement. Each of these considerations add conservatism to the
design. When the conservatism from each of these design considerations is
combined, the GRS retaining wall design can be grossly overconservative.
2.2.1 External and Internal Stability
The design consideration for external stability is satisfied when there is
an adequate safety margin for failure due to sliding, foundation bearing and
overall slope failure. Similar to the design approach for conventional
retaining walls, external stability is based on limit equilibrium analysis where
destabilizing forces (e.g., lateral earth pressure) against the reinforced soil
mass are resisted by stabilizing forces (e.g., reinforced soil mass weight and
external forces) with adequate margins for safety. Internal stability is satisfied
when the wall is sufficiently stable against failure within the reinforced soil
mass. External stability design methods are well understood and are
therefore not addressed in this study. However, internal stability design
methods for GRS retaining walls have not been well-established and can
vary from one design to another.
The retaining walls selected for this study were designed using a
commonly used design approach. In general, the internal stability of the
selected walls was satisfied using an ultimate-strength approach based on
the method of limit equilibrium. The ultimate-strength approach applies
18

factors of safety to the ultimate strength of the materials (i.e., soil,
reinforcement and facing) or to the computed quantities (i.e., forces and
moments) or to both the ultimate strength and calculated quantities (Wu,
1994b). The specific quantities and strength parameters include:
Lateral forces from the surcharge, reinforced soil mass and retained
soil;
Reinforcement tensile strength; and
Facing rigidity.
Due to the lack of reliable empirical data, somewhat arbitrary factors
of safety are used, which have resulted in overconservative designs. The
following subsections describe how the quantities, strength parameters and
associated factors of safety were determined for each project.
2.2.2 Lateral Forces
Lateral forces on a GRS retaining wall can be described by two
important characteristics. The first characteristic is the location of the failure
surface. The second is the lateral earth pressure distribution providing the
driving forces. As mentioned previously, these two characteristics are being
studied by others.
In general, the retaining wall designs in the selected projects assumed
a Rankine planar failure surface through the reinforced mass. The part of the
reinforcement that extends beyond the assumed failure wedge is
considered to be tension-resistant tiebacks (frequently referred to as the tied-
back wedge method) as illustrated on Figure 2.3. The tie-back wedge
method of analysis assumes that the shear strength of the reinforced soil
mass behind the wall is fully mobilized and thus active lateral earth pressures
are developed.
19

The second characteristic is the assumed lateral earth pressure
distribution. Typical lateral earth pressure distributions such as the linear
Rankine surface typically overestimate the lateral force on the reinforced soil
mass adding conservatism to the designs. Claybourn and Wu (1993)
compared six design methods and revealed that there are very significant
discrepancies in the factors of safety for various design methods due to
varying earth pressure distributions. In a typical wall examined in that study,
the combined factors of safety ranged from 3 to 23, depending on the earth
pressure distribution used. Typically, a linear Rankine lateral earth pressure
distribution was assumed for the selected projects. In most cases an active
condition was assumed. However, the Interstate Highway 70 through
Glenwood Canyon project design assumed at rest" conditions.
2.2.3 Reinforcement Tensile Strength
In the tie-back wedge method of analysis, the lateral earth pressures
are resisted by the tensile strength of the reinforcement. This is the design
component that is adjusted to account for creep since geosynthetics are
comprised of creep-sensitive polymers. The adjustments include reducing
the short-term tensile strength to account for creep and then further
reductions to account for construction damage and biological degradation.
The strength, adjusted for creep, is referred to as the creep-limited strength.
The creep-limited strength adjusted for construction damage and biological
degradation is referred fo as the design-strength. The short-term, creep-
limited, and design tensile strengths for the types of reinforcement used in the
selected projects are summarized in Table 2.2. Each type of reinforcement
strength is described in the following subsections
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2.2.3.1 Short-Term Strength
The short-term tensile strength of the geosynthetic reinforcement is
determined by applying a tensile load to an unconfined or confined test
sample at a constant strain-rate until failure occurs. During the loading
process, both load and displacement are measured to obtain a stress-strain
curve as illustrated on Figure 2.4.
The maximum tensile stress is typically referred to as the ultimate stress
or short-term stress. The strain at failure is typically referred to as the
maximum strain. Stress is typically measured in load per unit width and the
strain is computed by dividing the elongation by the original specimen
length. These values are illustrated on a typical stress-strain curve on Figure
2.4.
23

The American Society for Testing and Materials (ASTM) recently
standardized the procedure for determining the unconfined short-term
strength and maximum elongation for geosynthetics which is described in
ASTM Test Method D 4595, Tensile Properties of Geotextiles by the Wide
Width Strip Method". The ASTM D 4595 wide-width test uses a geosynthetic
sample that is 8 inches in width and 4 inches in gage length. The sample is
stressed uniaxially at a constant strain rate of 10 percent per minute until
failure occurs. The short-term strengths for the reinforcement used for the
Interstate Highway 70 through Glenwood Canyon, Norwegian Geotechnial
Institute, FHWA and Seattle Preload Fill Projects were determined by this
method.
The short-term strength for the Tanque Verde Wrightstown Pantano
Roads Project was determined using a four-inch-wide sample stressed
uniaxially at a constant rate of 2 percent per minute. The test method for the
short-term strength of the reinforcement used in the remaining two projects
(the Highbury Avenue and Japan Railway Test Embankment projects) were
not available in the literature. The smaller width sample used for the Tanque
Verde Wrightstown Pantano Roads project most likely produced a weaker
load-displacement response of the sample due to the Poisson effect (Wu
and Tatsuka, 1992) therefore adding conservatism to the design.
25

2.2.Z.2 Creep-limited Strength
The creep-limited strength values reported in the literature for the
selected projects are listed in Table 2.2. The CRC for the projects that
reported it in the literature are also listed. The CRC is computed using the
creep-limited strength and short-term strength as illustrated in Equation 2.1.
CRC = Tcreep/Tuit Equation 2.1
Where: CRC = Creep reduction coefficient
T creep = Tensile strength accounting for creep
Tutt = Short-term strength
As shown in Table 2.2, the CRC values used for the selected projects
range from 40 to 65 percent. For comparison, The AASHTO-AGC-ARTBA Joint
Committee Task Force 27 (AASHTO, 1990) recommends the following CRC
values for different polymer-type materials:
Polymer Type Creep Reduction Coefficient
Polyester 40%
Polypropylene 20%
Polyamide 35%
Polyethylene 20%
For example, the creep-limited strength for a reinforcement with a
short-term strength of 1,000 Ib/ft would be 200 Ib/ft using a CRC of 20
percent. The reinforcement materials used for the selected projects were
manufactured from polypropylene and polyester polymers. Although the
CRC values used in the selected projects where higher than the
recommended values (i.e., less conservative) the reinforcements exhibited
very small strains over extended periods of time as will be discussed in
Chapter 3.
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The creep-limited strength for the Tanque Verde Wrightstown -
Pantano Roads project was determined by McGown (1984). Rapid creep
tests were performed to determine the creep-limited strength for the geogrid
reinforcement used in the project. These tests consisted of developing
isochronous load-strain curves at varying temperatures, strain rates and loads
to determine a load below which rupture by a ductile yield was not likely to
occur. Isochronous curves can be used to determine the load in a
geosynthetic for a certain strain at a given time. The other projects arbitrarily
selected various creep reduction coefficients to account for creep instead
of performing actual element tests.
The current AASHTO design procedure recommends determining the
creep-limited strength by the following method. Controlled laboratory
creep tests are performed for a minimum duration of 10,000 hours for a
range of load levels on reinforcement samples. The samples are then tested
in the expected loading direction, in either a confined or unconfined mode,
and at an assumed in-ground temperature of 70 Fahrenheit. The test results
are then extrapolated to the required design life using the procedure
outlined in ASTM D 2837. From the creep test, two tensile loads should be
determined: the limit state tensile load (Tnmit), and the serviceability state
tensile load (Tservice). The limit state tensile load is defined as the highest load
level at which the log time creep-strain rate continues to decrease with time
within the design lifetime without inducing either brittle or ductile failure. The
serviceability state tensile load is defined as the load level at which total
strain will not exceed 5 percent within the design lifetime. The design lifetime
is typically 75 years. AASHTO recommends that critical walls be designed for
a 100-year lifespan (AASHTO, 1990).
27

Since these creep tests take an extended amount of time, the majority
of designers used the recommended default values listed above in Section
2.2.3.2. Using default CRC value results in using only 20 to 40 percent of the
reinforcment's short-term strength. Moreover, partial factors of safety for
construction damage, durability, and overall internal stability further reduce
the creep-limited strength to obtain the design-strength as described below.
2.2.3.3 Design Strength
The design strengths reported in the literature for the selected projects
are listed in Table 2.2. The design strength is the tensile strength of fhe
reinforcement used for design purposes. Most design methods use a partial
factor of safety approach to compute the design strength where the creep-
limited strength (i.e., Tsmit and/or Tservice) is adjusted to account for site-specific
conditions. The AASHTO-AGC-ARTBA Joint Committee Task Force 27 currently
recommends the following procedure using fhe partial factors of safety to
compute the design strength.
1. Compute the allowable long-term reinforcement tension based on
a limit state criterion given by:
Tai = Tiimit/FD*FC*FS
Where: Tai= Allowable long-term tension based on a limit state
criterion
Tiimit = Creep-limited strength based on a limit state
FD = Partial factor of safety for polymer durability
FC = Partial factor of safety for construcfion damage
FS = Overall factor of safety to account for uncertainties
in structure geometry, fill properties, reinforcement
properties and externally applied loads
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2. Compute the allowable long-term reinforcement tension based on
serviceability state criterion given by:
Tas= T$ervice/FC*FD
Where: Tas = Allowable long-term tension based on
serviceability criterion
Tservice = The allowable long-term tension based on a
serviceability state
3. The design strength should be the lesser of Tai and Tas.
The partial factor of safety for durability accounts for the degradation
of the geosynthetic reinforcement due to chemical and biological exposure.
In the absence of product-specific durability information, AASHTO
recommends that the FD should be between 1.10 and 2.0. The partial factor
of safety for construction damage accounts for damage (i.e., rips, punctures)
to the reinforcement during wall construction. In the absence of full-scale
construction damage tests, AASHTO recommends that the FC should
between 1.25 and 3.0. For permanent, vertically faced GRS retaining walls
the minimum overall factor of safety should be no less than 1.5 (AASHTO,
1990). The partial factors of safety used in the selected projects are
described below.
2.2.4 Partial Factors of Safety
2.2.4.1 Factor of Safety for Durability
None of the selected projects directly used a factor of safety for
durability. However, reinforcement samples were exhumed from the walls
built for the Interstate Highway 70 through Glenwood Canyon and the
Tanque Verde Wrightstown Pantano Roads projects located in Colorado
and Arizona respectively. Reinforcement samples were exhumed
29

approximately 11 years and 8 years after construction for the Colorado and
Arizona projects, respectively. After the samples were exhumed, they were
tested to determine their tensile strength and compared with the tensile
strength of archived samples cut from the same reinforcement material lots
used in construction. The Colorado project used a non-wooven geotextile
reinforcement manufactured from polypropylene and polyester polymers,
while the Arizona project used a geogrid reinforcement manufactured from
a polypropylene polymer.
The results from the durability testing indicate that the geosynthetic
material degrades very little over time in normal soil conditions. In both
projects, no significant decrease in tensile strength was observed in the
exhumed samples (Bright et al., 1994; and Bell and Barrett, 1994). For
comparison, the current factor of safety recommended by the Task Force 27
report (e.g., 1.10 to 2.0) reduces the creep-limited tensile strength of the
reinforcement by 10 to 50 percent.
2.2.4.2 Factor of Safety for Construction Damage
Similar to the factor of safety for durability (FD), the factor of safety for
construction damage (FC) was left out of the design computations for fhe
selected projects. The reinforcement samples exhumed from the Colorado
project exhibited an average 27 percent loss of strength based on element
tensile strength due to construction damage (Bell and Barrett, 1994) even
though the wall performed very well.
Similar to element tests for creep, the reduction in the element
strength due to construction damage represents only the behavior of the
reinforcement alone without accounting for fhe confinement of the
reinforced soil and soil/reinforcemenf interacfion. Recently, San and Matsui
30

(San and Matsui (____)) performed a test on a 20-foot-high wall where the
reinforcement embedded in the wall was cut using electrical wiring. The
reinforcement was cut at varying lengths starting from a distance furthest
from the face and progressing to the face of the wall. Each time the
reinforcement was cut, lateral and vertical displacements and
reinforcement strains were measured. After all the reinforcement layers had
been cut within approximately 1.5 feet behind the face, the total lateral
displacement was only approximately 1.5 inches. Based on the tie-back
wedge design concept, the wall should have collapsed once the
reinforcement was cut inside the Rankine failure surface. This test provides an
excellent illustration of the fact that neither construction damage or
degradation of geosynthetics will hinder its reinforcing function. Cutting the
geosynthetic reinforcement into small segments following construction can
be considered an extreme form of construction damage and
biological/chemical degradation. Apparently, whether the reinforcement is
continuous or not has little effect on the function of the reinforcement to
restrain lateral deformation of the soil.
The test performed by San and Matsui can provide reasons for the
good performance of GRS retaining walls even with construction damage
like in the Colorado project. From the test results and performance of the
selected case studies, two conclusions regarding the factor of safety for
construction damage can be made:
Element tensile strength tests on exhumed reinforcement does not
characterize the impact to a GRS retaining wall due to construction
damage; and
The recommended construction damage factors of safety (i.e., 1.25
to 3.0) are overconservative.
31

2.2 4.3 Overall Factor of Safety
The Seattle Preload Fill located in Washington, USA and the Tanque
Verde Wrightstown Pantano Roads Project located in Arizona, USA, used
overall factors of safety of 1.2 and 1.5 respectively in their designs. In both
cases, the walls performed very well. Since soil properties can vary, a
recommended overall factor of safety of 1.5 may be reasonable in GRS
retaining wall designs. By using a factor of safety of 1.5, the reinforcement
design strength is computed by reducing the short-term tensile strength by 33
percent.
2.2.5 Facing Rigidity
By placing geosynthetic reinforcing in the soil, the strength of the soil is
improved such that the vertical face of the soil/geosynthetic composite is
self-supporting; therefore, most designs ignore the resistance of the facing.
However, most GRS walls use facing for atheistic purposes and to prevent
raveling between the reinforcing elements. Most types of facing include
concrete modular blocks that are dry stacked in front of the wall. Other
types of facing materials include rigid concrete panels and wrapped
geosynthetics. The Seattle Preload Fill, Interstate Highway 70 through
Glenwood Canyon and Norwegian Geotechnical Institute projects used a
wrapped geotextile face as illustrated on Figure 2.2. and in the project
photographs in Appendix A. Shotcrete was placed on the Glenwood
Canyon project wall to prevent ultraviolet degradation of the geotextile.
Modular block type facing was used for the FHWA research project illustrated
on Figure 2.2 and in the project photographs in Appendix A.
The Japan Railway Embankment, Tanque Verde Wrightstown -
Pantano Roads and Highbury Avenue projects used rigid concrete panels. In
the latter two projects, the facing was mechanically attached to the
32

reinforcement. For these two projects, the reinforcement strains were highest
at the face than at other locations along the reinforcement. This is due to
larger settlement of the reinforced fill relative to the rigid facing (Bright, 1994;
and Bathurst, 1992). The Japan Railway project used a flexible concrete
panel on the middle section of the wall to compare the walls performance
using rigid and flexible facing material. The portion with the flexible facing
exhibited much larger deformation than the rigid facing (Tatsuoka, 1992).
33

3. Project Long-Term Performance
This chapter summarizes the performance of the GRS retaining walls
selected from the literature review and survey. The following section
describes the instrumentation and measured parameters used to quantify
the long-term performance of the walls. Section 3.2 provides the overall
performance of the walls including the reinforcement strains and wall
movements. Section 3.3 describes a conservatism index (Cl) that was
developed to quantify the conservativeness of the designs used in the
selected projects. Section 3.3.1 describes the creep modulus developed to
quantify the rate of creep.
3.1 Instrumentation and Measured Parameters
For each of the selected projects, instruments were installed during
construction to quantify the behavior of GRS retaining walls in field
conditions. The long-term performance was quantified by recording
instrument readings periodically over an extended period of time and
documenting the results in published papers. Specific behavior parameters
were monitored for each project depending on the projects objectives as
described in Chapter 2. In general, the behavior parameters listed below
were measured:
Horizontal and vertical displacements of the reinforced soil mass;
Reinforcement strains in selected layers and locations; and
External and internal soil temperatures.
Strain gauges were installed on selected layers of reinforcement at
varying distances from the face of the wall. The primary objective in most of
the projects was to determine the location of the maximum strain in the
reinforcement. This would confirm the theoretical location of the failure
34

surface assumed for design. The second objective was to measure the
magnitude of strain in the reinforcement during and after-construction. The
location and type of instrumentation used for each project are illustrated on
the project description sheets provided in Appendix A.
3.2 Reinforcement Strains and Wall Movement
The maximum reinforcement creep strain and wall movements for
each project are listed in Table 3.1. If the creep strain was unavailable in the
literature for a particular project, it was computed based on the incremental
change in total strain. Note, that the creep strain listed in Table 3.1 refers to
the deformation of the wall due to creep occuring after construction. The
movement listed in Table 3.1 refers to the total displacement of wall since
the beginning of construction. In some cases, the majority of the movement
was during construction. The CRC used for the design and recommended
by AASHTO for each project is also listed.
3.2.1 Interstate Highway 70 through Glenwood Canyon Project
The Interstate Highway 70 through Glenwood Canyon project was
purposely designed to determine the lower stability limits by designing at or
near the equilibrium factor of safety. It was anticipated that the
reinforcement would exhibit excessive strains on the order of 55 percent, yet
little movement within the reinforced soil mass was observed. Approximately
one year after the wall was constructed, a surcharge load was applied to
the top in an attempt to create failure conditions. The surcharge consisted of
a 15 foot high soil embankment applying a pressure of approximately 1,950
lb/ft2. However, failure never occurred.
The wall was constructed on a weak foundation soil and experienced
significant movement. The retaining wall experienced over two feet of
differential settlement from one end of the wall to the other due to
35

consolidation of underlying clays. Despite the large differential settlements,
only small strains occurred in the reinforcement (Derakhashandeh and
Barrett, 1986).
The CRC values used in the design of the wall ranged from 40 to 55
percent for reinforcement layers manufactured from polypropylene type
polymers and 65 percent for the reinforcement layers manufactured from
polyester type polymers. AASHTO recommends CRC values of 20 and 40
percent for polypropylene and polyester respectively (AASHTO, 1990). The
CRC values used for the design are over two and one and half times less
conservative for the polypropylene and polyester reinforcement layers
respectively, yet the wall performed very well.
Since the wall performed better than anticipated, the researchers
concluded that the mechanisms of geosynthetic reinforcement soil are not
well understood and the ability to select allowable loads is limited (Bell,
1983). They also concluded that more full-scale walls should be
instrumented and monitored to better understand the behavior of the
soil/geosynthetic interaction.
36

3.2.2 Tanque Verde Wrightstown Pantano Roads Project
The performance of Wall Panels 26-30 and 26-32 was monitored for
approximately seven years after construction. Geogrid reinforcement strains
were measured in the bottom, middle and top layers of the two wall panels
using resistance strain gages and inductance coils. Strain readings from the
inductance coils had a large variance due to low strains in the
reinforcement, therefore the results were believed to be unreliable (FHWA,
1989) so the readings from the strain gauges are reported in this study.
Reinforcement strains were measured during construction, two weeks
after construction and thereafter on an annual basis. The post-construction
strain measurements were adjusted to account pretensioning and
compaction during construction so that strains measured after construction
would be the result of creep.
The lateral movement of the wall was measured by surveying points
at the top of the wall. The points were surveyed during construction and up
to one month after construction. During construction, the top of the both
walls moved laterally approximatley three inches while the bottom of the
wall remained stationary. Little movement was observed after construction.
The mean total creep strain in the reinforcement after construction is
illustrated on Figure 3.1. As illustrated on Figure 3.1, the strain increased in the
reinforcement during the first year of service indicating that creep was
occurring. Thereafter, however, the creep strain remained generally
constant indicating that the wall had stabilized with time. The maximum
creep strain recorded was less than 1.0 percent. Based on isochronous load-
strain curves developed by McGown (1984), the load induced in the
38

reinforcement at 1.0 percent strain was approximately 265 Ib/ft. This is
approximately only 5 percent of the short-term strength (5,400 Ib/ft).
39

3.2.3 Norwegian Geotechnical Institute Project
The performance of the Noregian Geotechnical Institute project wall
sections J and N was monitored for approximately four years since its
construction. Both the force and strain was measured in the reinforcement.
Section N" had twice as many layers of reinforcement than Section 'J'.
Following construction, the wall was monitored for approximately four weeks
under self-weight loading. Thereafter, the top of the wall was cyclically
loaded for two months followed by a permanent surcharge.
The mean total creep strain in the reinforcement for the two sections
following application of the permanent surcharge loading is illustrated on
Figure 3.2. The creep strain was determined from the incremental increase in
the total strain begining 10 days after the surcharge was placed. The
maximum strain over the four years was approximately 0.5 and 0.6 percent in
section J and N respectively. The maximum tensile force in the
reinforcement after the permanent surcharge reported in the literature was
approximately 200 Ib/ft for both of the sections. This is approximately 6
percent of the short-term strength (3,600 Ib/ft). The CRC value used in the
design for this project was unavailable in the literature. The CRC value
recommended by AASHTO for the reinforcement type used in the two
sections is 20 percent (AASHTO, 1990).
41

3.2.4 Japan Railway Test Embankment Project
The performance of fhe Japan Railway Test Embankment JR Number
1 was monitored approximately two years since its construction. The vertical
and lateral displacement and tensile force in the reinforcement was
measured in three wall sections (cross sections D-D, F-F, and H-H) illustrated
on the project description sheets in Appendix A. Figure 3.3 illustrates the
monitoring results.
As illustrated on Figure 3.3, the tensile force in the reinforcement
increased during the first eight months reaching a nearly asymptotic state
similar to the performance of the other projects. The maximum tensile force
in the reinforcement was approximately 131 Ib/ft. This is approximately only 7
percent of the short-term strength (1,880 Ib/ft). The CRC value used in the
design for fhis project was unavailable in the literature. The CRC value
recommended by AASHTO for the reinforcement type used in the two
sections is 40 percent (AASHTO, 1990).
43

3.2.5 Highbury Avenue Project
The Highbury Avenue Wall was monitored for approximately two
years. Reinforcement strain was measured after the props holding the
concrete panels were removed. Reinforcement strain was measured
thereafter in December 1990; then in March 1990; July and August 1990; and
a year latter in August 1991. The creep strain was based on the incremental
change in the strain since December 1990. The maximum reinforcement
creep strain was approximately 1.5 percent based on the mean creep strain.
The mean creep strain over time is illustrated on Figure 3.4. Similar to the
previous projects, the wall exhibited creep over the monitoring period,
however, had begun to stabilize with time. The CRC value used for the
project was unavailable in the literature.
3.2.6 Federal Highway Administration Research Project
Wall nine built for the FHWA project was monitored for approximately
one year. Reinforcement strain and total wall movement was recorded
more frequent then the previous projects. Instrument readings were
recorded on an almost daily basis during construction and during placement
of the surcharge load. The surcharge was completed November 10, 1989.
Thereafter instrument readings were recorded nine times up through
November 11, 1990.
The maximum creep strain computed after the surcharge load was
placed is illustrated on Figure 3.5. The creep strain was based on the
increament increase in total strain. As illustrated on Figure 3.5, the creep
strain shows that the wall was becoming stable with time. The maximum
creep strain was approximately .7 percent over the one year monitoring
period. The total lateral movement after the props were released was
45

approximately 3.6 inches. The measurement was based on the vertical
inclinometer directly behind the face of the wall. Most of the movement is
most likely due to the tensioning of the reinforcement.
The CRC value used for the design was 60 percent. AASHTO
recommends a more conservative CRC of 40 percent for the type of
reinforcement used in wall nine. Although the CRC value used in the design
was one and half times higher (e.g., less conservative), the reinforcement
strains were very small.
3.2.7 Seattle Preload Fill Project
The southeast wall for the Seattle Preload Fill project was monitored for
approximately one year after its construction. Similar to the FHWA wall,
instrument readings were recorded on a frequent basis. The maximum
reinforcement creep strain in the reinforcement over time is illustrated on
Figure 3.6. Creep strain was determined immediately after the surcharge
was placed on the wall. As illustrated on Figure 3.6, creep was occurring
and beginning to stabilize. The maximum creep strain recorded in the
reinforcement was less than 0.5 percent.
The CRC values used for the design were 40 and 60 percent for
polypropylene and polyester type reinforcement respectively. AASHTO
recommends CRC values of 20 and 40 percent for polypropylene and
polyester respectively. The CRC values used were two and one and half
times less conservative than the recommended values, yet very little strain
was observed in the reinforcement.
The researchers concluded that the low strain were the result of lower
than expected load level in the reinforcement or due to poorly understood
interaction between the reinforcement and the confining soil. Additionally,
46

the reinforcement was damaged during construction damage with no
apparent impact to the performance.
47

3.3 Conservatism Index
The selected GRS retaining walls vary from conservative to less
conservative designs. For example, the Interstate Highway 70 through
Glenwood Canyon and Section 'J' of the Norwegian Geotechnical Institute
projects were purposely designed to determine the lower stability limits by
designing at, near equilibrium or even below factors of safety. Conversely,
the Highbury Avenue and Tanque Verde Wrightstown Pantano Roads
projects were designed using more conservative assumptions.
Due to the variability in retaining wall designs, direct comparison of
the selected projects would be misleading. Therefore, a conservatism index
(Cl) was developed so that the design of the walls could be evaluated. In
general, the Cl value is computed using a limit equilibrium analysis where
resisting lateral force provided by tensile strength of the reinforcement is
divided by the driving lateral force of the earth. The Cl value is based on the
same principles of limit equilibrium used in the current design methods where
the resisting tensile force is entirely provided by the reinforcement and
redistribution of stresses due to the soil/geosynthetic interaction are ignored.
The Cl value takes into consideration the reinforcement strength, number of
reinforcement layers, and active lateral earth pressure caused by the
retained soil and surcharge. These parameters and the resulting Cl for each
project is listed in Table 3.2. Detailed computations are provided in
Appendix B.
51

The Cl is an index value to indicate the relative conservativeness of a design.
Similar to a factor of safety concept, a Cl value close or less than one is
considered a less conservative design. A design with a greater Cl value is
more conservative relative to a design with a smaller Cl value. As an
example, if project A has a Dl value of 3 and project B as a Dl value of 5,
theoretically, project B should perform better (i.e., smaller displacements and
strains) than project A.
The Cl for the selected projects ranged from 0.4 to 8.7. The less
conservative designs have a Cl of 0.4 and include the Interstate Highway 70
through Glenwood Canyon project and wall section 'J' of the Norwegian
Geotechnical Institute project. Both these walls were purposely designed
using less conservative assumptions, however still performed very well. Since
none of the selected projects exhibited large strains, it is difficult to correlate
the Cl value with an under designed GRS retaining wall to determine the
lower bound Cl. However, a Cl value greater than 0.4 would indicate a
more conservative design since the walls with the lower Cl values
demonstrated good long-term performance.
The Tanque Verde Wrightstown Pantano Roads project wall panels,
Highbury Avenue and Federal Highway Administration projects have Cl
values ranging from 7.9 to 8.7. Each of these projects used high tensile
strength reinforcement ranging from 2,000 Ib/ft to 5,400 Ib/ft (short-term
strength). For comparison, the Interstate Highway 70 through Glenwood
Canyon wall used reinforcement layers with a short-term tensile strength of
220 Ib/ft.
3.3.1 Creep-Rate and the Creep Modulus
Creep-rate is the time-rate at which a GSR retaining wall deforms
under a sustained load. The change in the creep-rate can be used to
53

quantify the stabilization of a GRS retaining wall due to creep. A constant
creep-rate would indicate that the wall is deforming at a constant rate
which would be considered secondary creep. An increasing creep-rate
would indicate that the wall is deforming at an increasing rate which would
be considered tertiary creep.. In either cases, the wall could conceivably
reach a creep failure condition. Conversely, a decreasing creep-rate would
indicate that the wall was stabilizing with time reaching an equilibrium
condition.
Figure 3.8 illustrates the creep rates computed for the selected
projects. As illustrated on Figure 3.8, there is a decreasing trend in the creep-
rates indicating the GRS walls were stabilizing over time. This behavior has
also been observed in laboratory soil/geosynthetic composite creep tests
conducted by Ketchart and Wu (1996). Moreover, the decreasing trends
were close to linear when plotted on logarithmic scale. The slope of the
linear relation is referred to as the creep modulus (CM). The CM is illustrated
in the example below on Figure 3.7.
Figure 3.7: Creep-rate-Time Curve Illustrating the Creep Modulus
54

The CM value provides a means to characterize the long-term performance
by quantifying the slope of the creep-rate time curve The CM computed for
each project are listed in Table 3.3. .The regression lines used to compute the
CM are illustrated on the Figures C.l through C.5 in Appendix C. The CM for
the selected projects range from 0.57 to 1.13 %/day2. This is a fairly narrow
range given the wide variety of retaining wall types in the study. The
decreasing slope in the creep-rate and similar slopes were also observed in
the laboratory tests performed by Ketchart and Wu (1996). The CM may be a
good parameter to characterize the creep behavior of the soil/geosynthetic
interaction which will be discussed in Chapter 5
Based on the CM, the creep-rate for the selected projects are decreasing at
a rapid rate indicating that the walls are stabilizing with time. Moreover, it
demonstrates the arbitrary nature of reducing the reinforcements short-term
strength by up to 80 percent using element creep tests, CRCs and/or partial
factors of safety.
56

4. An Approach to Estimating Creep Using a Laboratory Test
Chapter 3 demonstrated that the current design methods significantly
over-estimate the magnitude of strain in the geosynthetic reinforcement and
movement of the wall face caused by creep. However, the longest period
of performance data for any of GRS retaining wall is less than 10 years. Most
applications require that permanent retaining walls be designed for a
minimum service life of 75 to 100 years (AASHTO, 1992). Thus, a rational
means for estimating creep based on the soil/geosynthetic interaction is
needed. This Chapter describes an approach for estimating creep using a
simple laboratory test and analytical solution.
4.1 Creep in Laboratory Tests
Development of the method begins with evaluating recent laboratory
tests conducted at the University of Colorado at Denver to determine creep
behavior of soil/geosynthetic composites. Wu (1994a) and Wu and Helwany
(1996) developed a laboratory test procedure to characterize the
soil/geosynthetic composite behavior. The apparatus used in for the
procedure allows the stresses applied to the soil to be transferred to the
geosynthetic reinforcement in a manner similar to typical GSR walls. Using
the device, they conducted two long-term performance tests, one using a
clay backfill and the other using a sand backfill. A second study was
performed In 1995 by Ketchart and Wu (1996). They simplified the testing
apparatus device and performed tests on various soils and geosynthetics
under different conditions, including accelerated creep tests at elevated
temperatures.
58

In both studies, it was observed that the long-term deformation
behavior of the soil/geosynthetic composite was significantly affected by the
time-dependent behavior of the soil and the geosynthetic reinforcement. In
general, if the confining soil has a tendency to creep faster than the
geosynthetic reinforcement, the geosynthetic will impose a restraining effect
on the deformation of the soil through friction and/or adhesion between the
two materials. Ketchart and Wu (1996) observed that in each case with
granular soil, the creep rate decreased over time. This behavior was also
observed in the reinforcement strains for the full-scale walls described in
Chapter 3. The following subsections describe the laboratory test, the test
results and the procedure developed to estimate creep deformation over
the design life of a GRS wall.
4.1.1 Laboratory Creep Test Descriptions
The test apparatus developed by Wu and Helwany (1996) consists of a
Plexiglas box with thin sheet-metal sides approximately 1.5 feet by 3 feet in
size. A layer of reinforcement is sandwiched between two soil blocks placed
inside the box using techniques similar to field construction procedure. Then
the composite is loaded with a sustained surcharge load. The side-wall
adhesion between the Plexiglas and the soil was minimized by creating a
lubrication layer at the interface of the two materials to create plain strain
conditions. The test apparatus is illustrated on Figure 4.1
One of Wu and Helwanys tests consisted of placing an Ottawa sand
into the testing apparatus using a air-pulviation method. Once half the sand
was placed, a layer of geotextile was placed and securely attached to the
two sheet metal plates, followed by the remaining sand. Another layer of
geotextile was then placed at the top of the sand. The soil/geosynthetic
59

composite was loaded with a sustained vertical load of approximately 16
lb/in2 for 30 days. The stress-strain behavior of the geotextile was determined
by performing a series of element geotextile creep tests to compare its
behavior with the behavior of the soil/geosynthetic composite. Lateral and
vertical deformation and reinforcement strain were measured over the
testing period. The test results indicate that the element creep test over-
estimated the strain in the reinforcement by a factor of four consistent with
the performance of the full-scale retaining walls described in Chapter 3.
Ketchart and Wu modified the apparatus developed by Wu and
Helewany so that the lateral supports could be released to model "worst"
case conditions. This would be similar to removing the modular blocks or
other type of facing from the front of a GRS retaining wall, exposing the soil
and the reinforcement. Similar to previous test procedures, soil was placed
in the test apparatus and compacted to mid-height. A layer of geotextile
reinforcement was then placed (without attaching to the side walls)
followed by compacted soil to the top of the apparatus. The sample was
then subjected to a sustained surcharge for a period of 30 days. In some
cases, the apparatus was placed in a room with elevated temperatures to
accelerate creep of the geosynthetic. The test apparatus is illustrated on
Figure 4.2.
60

The types of tests performed during Ketchart and Wu's study are
described below.
A total of 11 tests were performed during the study. From the testing
program, six of the tests conducted using a granular backfill is of interest for
this report. These include the tests described below (Ketchart and Wu, 1996).
Test D-l: Test D-l was performed using a heat-bonded nonwoven
polypropylene low-strength geotextile having a short-term tensile
strength of 420 Ib/ft. and an average vertical pressure of 15 lb/in2
at a temperature of 70 F. The test was performed to determine
the creep behavior of the soil/geosynthetic composite using a low-
strength reinforcement. Reinforcement strain was measured in
addition to lateral and vertical displacement.
Test H-1: Test H-1 was performed using a woven geotextile having
a short-term tensile strength of 4800 Ib/ft and an average pressure
of 30 lb/in2- at a temperature of 125F. The test was performed to
determine the creep behavior of the soil/geosynthetic composite
using a large load at an elevated temperature.
Test R-l: Test R-l was performed using a woven geotextile having a
short-term strength of 4800 Ib/ft and an average vertical pressure of
15 lb/in2 at a temperature of 70 F. The test was performed to
determine temperature effects on the creep behavior of the
soil/geosynthetic composite by comparing the results with test R-2.
Test R-2: Test R-2 was performed using the same material and
loading as R-l except at an elevated temperature of 125F. The
test was performed to determine temperature effects on creep
behavior of the soil/geosynthetic composite by comparing the
results with test R-l.
Test R-3: Test R-3 is a duplicate test of R-2 to determine the
repeatability of the test method.
Test W-l: Test W-l was performed using a woven geotexitle having
a short-term tensile strength of 1440 Ib/ft and an average pressure
of 15 lb/ in2 at an elevated temperature of 125F. The test was
performed to determine the temperature impacts to the creep
63

behavior of the soil/geosynthetic composite using a low strength
reinforcement.
The granular backfill consisted of a road base comprising of a silty
sandy gravel. The soil was prepared 2 percent wet of optimum moisture
content and compacted to 95 percent of the relative density or
approximately 126 lb/ft3 having an internal friction angle of 34.
4.1.2 Laboratory Test Creep Rate
Ketchart and Wu measured lateral and vertical displacements and in
one test, strain in the reinforcement due to creep. Lateral displacements
were measured using linear voltage deformation transducers installed at the
mid-height of the testing apparatus where the reinforcement was located.
Strain was measured in the reinforcement for test D-l only. The lateral
displacement over the time period for each of the above tests are plotted
on Figure 4.3. From the lateral displacement data, the lateral creep rate was
computed and plotted on Figure 4.4. The creep rate based on the
measured maximum strain in the reinforcement for test D-l is also shown.
As illustrated on the Figure 4.3, the effects of geosynthetic strength,
temperature, and loading all impact the time-dependent behavior of the
soil/geosynthetic composite as to be expected. However, there is a linear
decreasing trend in the creep rates of all the tests when plotted on a
logarithmic scale as illustrated on Figure 4.4. After performing a linear
regression on each of the data sets, the confidence (R-squared) coefficient is
on the order of 94 percent demonstrating a good linear fit. Moreover, the
slopes of the linear relation or CM are approximately the same for all the
tests.
64

4.2 Laboratory and Full-Scale Creep Rate Comparison
Figure 4.5 illustrates the creep rates computed tor the selected full-
scale projects and the creep rates computed from the lateral displacement
of the laboratory tests. It is observed that the full-scale data fits well with the
laboratory data and shows a continuing decreasing trend in the creep rate.
Moreover the CM for the selected projects and the laboratory tests are
nearly the same. The CM values are listed in Table 4.1. The plots used to
compute the CM values are provided in Appendix C.
The full-scale creep performance also demonstrates the validity of the
testing procedure developed by Ketchart and Wu by accurately modeling
the soil/geosynthetic integration of a full scale GRS retaining wall with
granular soil. The full-scale creep performance also demonstrates that the
laboratory procedure can determine the creep behavior of a
soil/geosynthetic composite material consisting of granular soil in a relatively
short amount of time unlike the 10,000 hour element creep tests currently
required.
67

4.3 An Analytical Solution for Estimating Creep Strain
The previous sections demostrated that the creep-rate for the
laboratory soil/geosynthetic composite tests and full-scale walls could be
represented as a straight line when plotted on a lograthimic scale. To
determine the creep strain at any given time, the strain-rate can be plotted
with time as illustrated in Figure 4.3.
TIME (t/t0)
Figure 4.3: Creep-Rate-Time Ratio Plot
70

Using the plot illustrated on Figure 4.3, the linear relationship can be
represented by the following equations:
The creep-rate coefficient (A) is the creep-rate corresponding to a t/t0
value of one. The reference time (to) is the time at which the creep-strain
begins. Typically this would be at the end of construction of a wall. For
example, if it took 30 days to complete the construction of the wall, t0 would
be 30 days.
Integrating Equation 4.2 gives the creep-strain expressed in Equation
4.3:
Equation 4.1
or,
Equation 4.2
Where: dt = Creep-rate (%/day)
m = Slope of the log (t/t0) vs. log (dsc/dt) curve
A = Creep-rate coefficient (%/day)
t = Time (day)
to = Reference time (day)
1 m
+ c
Equation 4.3
Where: C = Integration constant
71

Equation 4.3 can be solved by two unique solutions. Knowing that the
-*t0
creep strain (sc) is zero at t = to, C will be equal to 1 ~ m when the slope (m)
is not equal to 1. When the slope (m) is equal to 1, C equals zero. Thus, the
analytical solution for determining creep strain at a given time can be
expressed by Equations 4.4 and 4.5:
A-t,
To /t\1_m A-t0
1 -m w
1 m
When: m 1
Equation 4.4
Â£c:= A- t0ln )
\To/
When: m = 1
Equation 4.5
Note that Equations 4.4 and 4.5 are only valid for a soil/geosynthetic
composite that exhibits a constant value of m. The smaller the m value, the
larger the creep-strain.
72

5. Summary and Conclusions
5.1 Summary
The three main research objectives ot this study included:
1. Compile long-term performance data from field projects involving
well-instrumented GRS retaining walls;
2. Develope a means to quantify the conservativeness of the designs;
and
3. Develop a rational method to estimate creep based on laboratory
creep test of the soil/geosynthetic composite deformation.
The first objective was accomplished by surveying experts in GRS
technology and performing an extensive literature search. From the survey
and literature search, seven well-documented GRS retaining wall projects
around the world were described and analyzed.
The second objective was achieved by showing that walls designed
using a CRC that was greater than the CRCs recommended by AASHTO (i.e.,
less conservative) performed exceptionally well under a variety of
conditions. The Cl was develop to provide a measure of conservativeness in
the designs. Even with a low Cl for some of the projects, the walls performed
exceptionally well.
The third objective was achieved by developing a simple procedure
for estimating creep based on the observed decreasing creep-rate of the
soil/geosynthetic composite. By using the simple testing procedure
developed by Ketchart and Wu and the analytical solution, the creep can
be predicted for any given time after construction for project specific soil
and reinforcement types.
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5.2 Conclusions
From the study, the following conclusions can be made:
1. GRS retaining walls with granular backfill deform very little due to creep:
The GRS retaining walls selected for the study represent a variety of
wall types using granular backfill and field conditions. The maximum creep-
strain in the reinforcement were less than 1.5 percent.
2. The actual reinforcement load is over-estimated:
In some of the selected walls, the tensile load in the reinforcement
could be estimated. In all those cases the tensile load was less than 10
percent of the reinforcements short-term strength. This suggests that the
design strength required for the reinforcement is too large
(overconservative). The design strength is the result of overconservative
creep reduction coefficients (CRCs) and partial factors of safety required by
the AASHTO design method. This results in limiting the type of reinforcement
in GRS walls to only higher-cost, high-strength geosynthetics.
3. The GRS retaining walls were stabilizing with time:
In all of the selected walls and laboratory tests, the creep-rate was
decreasing with time indicating that the walls were stabilizing with time. The
tensile forces in the reinforcement are likely to decrease with time as the
creep strain-rate becomes very small (known as "stress relaxation").
4. A simple laboratory test and analytical equation can be used to predict
creep:
It was observed that the logarithmic creep-rate for the full-scale walls
and laboratory soil/geosynthetic composite tests decrease in a linear
relationship with logarithmic time. From this observation, an analytical
equation can be used to predict creep during the design-life of a GSR wall.
In full-scale applications, the simple test developed by Ketchart and Wu may
be performed to determine the creep modulus of a project specific
geosynthetic/soil composite. Long-term creep deformation of the wall can
then be determined in a rational manner by the analytical solution.
5.3 Recommendations for Future Study
A wide variety of GRS retaining walls are being studied based on the
literature search and survey. However, the focus of the research seems to be
in several directions. The overall direction of the projects selected for this
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study was mainly to demonstrate the functionality of geosynthetic soil
reinforcement and that it basically "works". The projects selected can be
considered the first full-scale GRS retaining walls in field conditions that have
been monitored for extended period of times. Future research in monitoring
the performance of full-scale walls should be focused in the areas of lateral
earth pressure distribution, location of the failure surface and creep so that
specific data is collected to better understand the complex behavior of the
soil/geosynthetic composite.
Future research is required to determine the impact of material types
and the environment on the creep-rate relationship used to predict creep-
strain. Eventually, a database of creep-rate-time curves for specific
soil/geosynthetic composites could be established so that the magnitude of
creep can be estimated using analytical solutions.
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Monitored E5EE -!;r
Notes
a) Excerpted from the paper entitled "Instrumented field performance of a 6 m geogrid soil wall", (Simac, 1990).
b) The short term strength is excerpted from Simac (1990). Assumed to be determined based on the wide width tensile strength test
(ASTM D-4596).
c) The long term strength is excerpted from Simac (1990). Assumed to be Determined using 10,000 hour creep tests described in the
Federal Highway Administration's task force 27 report (FHWA, 1969).
d) The design strength is determined by dividing the long term strength by the factor of safety.
e) The factor of safety accounts for long term durability and construction site damage described in the Federal Highway
Administration's task force 27 report (FHWA, 1969).
82

Phil Crouse
Conservatism Index
Calculation Brief
4/19/96
Purpose:
The purpose of this calculation brief is to determine the conservatism index (Cl) for the
selected projects.
Methodology:
The Cl value is computed using a limit equilibrium analysis where resisting lateral force
provided by tensile strength of the reinforcement is divided by the driving lateral force of
the earth. The Cl value is based on the same principles of limit equilibrium used in the
current design methods where the resisting tensile force is entirely provided by the
reinforcement and redistribution of stresses due to the soil/geosynthetic interaction are
ignored.
The Cl value is based on the average lateral force (F) acting on the reinforcement layers
assuming a linear Rankine active pressure distribution. The Cl value is computed by
dividing the short-term tensile strength of the reinforcement by the average lateral earth
pressure acting on the wall. If the wall has different reinforcement spacings or
strengths, a weighted Cl value is computed. The computation is illustrated below
followed by a summary of the results and detailed computation for each selected
project.